Plant-eating, long-necked giants called titanosaurs were the largest vertebrates to ever walk the planet. Now, researchers studying the fossils of a tiny titanosaur from Madagascar reveal that the baby already had adult-like proportions. This growth strategy likely allowed the little giants to be more independent than other baby dinosaurs, according to a new Science study published this week. 

The titanosaur Rapetosaurus krausei reached immense lengths of about 15 meters (50 feet). However, they began life tiny: After hatching from eggs smaller than soccer balls, the infants were less than a meter long (3.3 feet) and weighed about 10 kilograms (22 pounds). Several Rapetosaurus fossils have been unearthed and analyzed, including eggs and juveniles, but little is known about their recently hatched babies. We just haven’t found many titanosaurs that were fossilized at their smallest stage. 

^{The preserved bones of UA 9998. Femur, tibia, right and left fibulae, right and left humeri, a metatarsal, metacarpal, phalanx, pubis, and a few partial vertebrae. K. Curry Rogers, M. Whitney, M. D'Emic, and B. Bagley}

A team led by Kristina Curry Rogers of Macalester College in Minnesota analyzed a new tiny Rapetosaurus specimen (UA 9998) from the Upper Cretaceous Anembalemba Member of the Maevarano Formation. The fossils include parts of leg and arm bone, girdle, and vertebrae, and their general appearance was indistinguishable from those of larger Rapetosaurus specimens – just in miniature.

This youngster likely died from drought-related starvation when it was between 39 and 77 days old. It weighed an estimated 3.4 kilograms (7.5 pounds) when it hatched, and by the time it died, it had grown to about 40 kilograms (88 pounds) and had a hip height of just 35 centimeters (14 inches).

Using X-ray computed tomography and by analyzing bone tissue, the team was able to study the dinosaur’s growth rate and pattern. Judging from the compactness of the bones, the researchers think its limbs stayed the same shape throughout its life. It could have fended for itself, and it was already actively walking around too. 

This so-called precocial growth strategy allowed the baby to move around independently immediately after it was born. In many other dinosaur groups – such as the carnivorous theropods (like T. rex) and the herbivorous ornithischians (like stegosaurs) – limb proportions change between birth and adulthood. For these other nest-bound baby dinosaurs to survive, parental care was a must. 

^{Image in the text: The femur of UA 9998 is in orange. The gray silhouette represents the size of a hatchling, and the other femora belong to a juvenile and an adult. K. Curry Rogers, M. Whitney, M. D'Emic, and B. Bagley}